MONDAY, Aug. 27 (HealthDay News) -- Women who are overweight or obese when diagnosed with the most common form of breast cancer have a higher risk of recurrence than slimmer patients, according to a new analysis.

"Patients who are obese are significantly more likely to have a breast cancer recurrence despite optimal therapy," said study leader Dr. Joseph Sparano, associate chairman of oncology at Montefiore Medical Center, in New York City.

These overweight and obese women with hormone receptor-positive breast cancer -- which accounts for two-thirds of all cases worldwide -- are also at greater risk of dying compared to normal-weight women, he found.

The comparison found the obese women had "about a 30 percent increased risk of recurrence and 50 percent increased risk of death," Sparano said.

Hormone receptor-positive breast cancer requires estrogen to grow, and Sparano speculated that one possibility for the link might be increased production of estrogen in heavier women.

While the new study findings, published online Aug. 27 in the journal Cancer, echo some earlier research, there are important differences, Sparano said.

Other studies have been complicated by the fact that obese patients often have additional health problems, which may have prevented them from getting appropriate therapy, Sparano said.

In the new analysis, he looked at a relatively healthy patients, 6,885 women in all, enrolled in U.S. National Cancer Institute clinical trials with stage 1 to 3 breast cancer (cancers that had not spread to distant sites). They could not participate if they had serious health problems, such as liver, heart or lung disease.

The percentage considered obese (body mass index of 30 or higher) ranged from 25 to 37 percent in the three studies evaluated. Body mass index (BMI) is a calculation of fat content based on height and weight.

As BMI rose, the risk of recurrence or death increased incrementally. Being overweight but not obese also increased risk.

"Obesity is clearly associated with an increased risk, and even women who were not obese but overweight tend to have a high risk," Sparano said. However, he said he can't pinpoint a weight threshold at which the risk begins.

It's important to note that although a link between recurrence and excess weight was detected, the research does not show cause and effect.

Commenting on the study, Dr. Bette Caan, a senior research scientist at the Kaiser Permanente Division of Research in Oakland, Calif., said the researchers were able to address some of the shortcomings of previous studies.

"To me the big news is that it is showing obesity is only related to one subtype," Caan said. "It's the most common subtype.

"Breast cancer is not one disease but several diseases," she added.

What is not yet known, agreed Sparano and Caan, is whether reducing weight after cancer treatment might reduce the risk of recurrence or death.

Studies looking at the effect of reducing dietary fat on the risk of recurrence in obese women have come up with conflicting results. In one study, reduced fat didn't lead to weight loss or reduced recurrence.

More study is needed, these experts say. It might turn out that weight loss only plays a role in reducing recurrence risk for some women, Caan said.